First, you would likely get more replies, asking this question in the coding sub-forum: http://www.v7n.com/forums/coding-forum/
A frameset is a way to split up a webpage, in forexample 3 pages on the same page as my example below. A iframe is often used to display content on a webpage from another webpage, forexample advertisements or youtube videos.
Anyway, a website using frames, usually relies on all the frames being present. So the website would most likely not work or have a limited amount of features if a user just viewed the url of one of these frames.
Lets say, the website have 3 frames, a top frame where it displays some advertisement, a left frame with some navigation links and a right frame with some content. When the user clicks a link in the left frame, another page is displayed in the right frame. Lets say, the ad-frames url is http://www.domain.com/ad.html, navigation frames url is http://www.domain.com/menu.html and the right frames url is http://www.domain.com/content.html.
Now, if Googlebot indexed those frame urls and an user found one of these frame urls in the search results, and clicked on it. Then the user would only see either a menu with no frame to display the content in and no advertisement(the site would die, since it don't make money ;-)), a advertisement with no navigation, or other content or some content without navigation or advertisement. So crawlers only index the page containing the frameset, so the page looks correct.
So we can conclude, it would be no problem for crawlers to index those frame urls, it's just a very very bad idea, so thats why the don't do it. And it's usually a bad idea in general to use framesets, because crawlers don't index the urls of the individual frames ;-)